It is frequently necessary to bend (i.e. plastically deform) relatively large metal workpieces, e.g. metal plates or sheets, utilizing a bending press into round configurations, i.e. configurations conforming approximately to that of a circle segment, e.g. a semicylinder.
Various bending dies are customarily provided for this purpose and the bending press may be capable of operating with such dies utilizing extremely high pressures generated, for example, hydraulically.
When large-diameter workpieces of circular segmental configuration are to be fabricated from steel plate or sheet, for example, the bending may be carried out stepwise by bending the workpiece in sections between two fixed spaced-apart supports on a bending anvil which can constitute the fixed member of the bending die.
The other member of the bending die is generally a ram which has a bearing line lying centrally between the bearing lines along which the workpiece is supported.
The bending ram is vertically displaceable and, during its downstroke, performs the bending action by depressing the workpiece between the two supports. The workpiece is then advanced transversely to the bearing lines upon retraction of the ram which, upon a subsequent descent, bends another section of the workpiece so that ultimately an approximately round curvature is imparted thereto.
Workpieces can be bent in this manner to form, for example, trough-shaped and even semicylindrical bodies which can be welded together to form large-diameter pipes, containers, tanks and the like.
The bending ram of prior art systems generally has a semicylindrical configuration, i.e. a semicircular cross-section, so that it engages with more or less point contact (as seen in cross-section) the workpiece in the geometric center between the two support points of the anvil.
As a result of this relationship, the bending action tends to progress outwardly from the contact point or line of the ram and to alter as the workpiece contacts more of the rounded lower end of the latter or as the bending causes the metal workpiece to roll along the surface of this end during the bending operation.
Experience has shown that this results in a continuous change in the curvature to either side of the central point of attack and thus in a bent section whose curvature is highly non-uniform. The greatest radius of curvature lies directly beneath the ram while the radius of curvature decreases to either side thereof so that, with reciprocation of the ram and advance of the workpiece in the manner described, a multiplicity of nonuniform curvatures can be imparted to the workpiece.
It is thus difficult, if not impossible, to obtain an approximately circular curvature within narrow tolerances as may be required for the fabrication of pipes, tanks or the like by welding such workpieces together.
Even when efforts are made to overcome this problem by reducing the feed increment of the workpiece to a minimum, the problem is not fully resolved and, of course, the reduction in the increment of advance of the workpiece during each stroke of the ram results in an extremely costly and time consuming process.